Formation of embedded micro-lens

ABSTRACT

Provided is an image sensor device. The image sensor device includes a pixel formed in a substrate. The image sensor device includes a first micro-lens embedded in a transparent layer over the substrate. The first micro-lens has a first upper surface that has an angular tip. The image sensor device includes a color filter that is located over the transparent layer. The image sensor device includes a second micro-lens that is formed over the color filter. The second micro-lens has a second upper surface that has an approximately rounded profile. The pixel, the first micro-lens, the color filter, and the second micro-lens are all at least partially aligned with one another in a vertical direction.

BACKGROUND

Semiconductor image sensors are used for sensing light. Complementarymetal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupleddevice (CCD) sensors are widely used in various applications such asdigital still camera or mobile phone camera applications. These devicesutilize an array of pixels in a substrate, including photodiodes andtransistors, that can absorb radiation projected toward the substrateand convert the sensed radiation into electrical signals.

Semiconductor image sensors use micro-lenses to focus incoming light. Amicro-lens can be formed in an embedded manner inside a layer of theimage sensor. In that case, an external micro-lens may also be used tocomplement the embedded micro-lens. Having both an external lens and anembedded lens may be desirable because they improve light sensitivity,for example sensitivity to black and white light. However, traditionalmethods of forming the embedded micro-lens are complex and timeconsuming. The embedded micro-lenses formed by the traditional methodsalso tend to have poor light-focusing performance.

Therefore, while existing methods of fabricating embedded micro-lensesfor image sensors have been generally adequate for their intendedpurposes, they have not been entirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method for fabricating asemiconductor device according to various aspects of the presentdisclosure.

FIGS. 2-6 are diagrammatic fragmentary cross-sectional side views of asemiconductor device at various stages of fabrication according tovarious aspects of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Various features may be arbitrarily drawn indifferent scales for the sake of simplicity and clarity.

Illustrated in FIG. 1 is a flowchart of a method 20 for fabricating afront-side illuminated (FSI) image sensor device according to variousaspects of the present disclosure. Referring to FIG. 1, the method 20begins with block 22 in which a radiation-sensing element is formed in asubstrate. The method 20 continues with block 24 in which a patterneddielectric layer is formed over the substrate. The patterned dielectriclayer includes a plurality of dielectric portions that are separated bya plurality of openings. The method 20 continues with block 26 in whicha laser annealing process is performed on the patterned dielectric layerin a manner such that each of the dielectric portions are melted andre-shaped. The re-shaped dielectric portions each have a pointy tip.

FIGS. 2 to 6 are diagrammatic fragmentary cross-sectional side views ofa FSI image sensor device 30 at various stages during its fabricationaccording to various aspects of the p resent disclosure. It isunderstood that FIGS. 2 to 6 have been simplified for a betterunderstanding of the inventive concepts of the present disclosure.

Referring to FIG. 2, the image sensor device 30 includes a substrate 40,also referred to as a wafer. The substrate 40 is a silicon substratedoped either with a P-type dopant or with an N-type dopant. The P-typedopant may be boron, and the N-type dopant may be phosphorous orarsenic. The substrate 40 may include other elementary semiconductorssuch as germanium. The substrate 40 may optionally include a compoundsemiconductor and/or an alloy semiconductor. Further, the substrate 40may include an epitaxial layer (epi layer), may be strained forperformance enhancement, and may include a silicon-on-insulator (SOI)structure.

The substrate 40 has a front side 45 and a back side 46, which are alsothe front and back sides of the image sensor device 30, respectively.The image sensor device 30, when completed, will sense or detectincoming radiation waves projected toward the substrate 40 from thefront side 45. In other words, the radiation waves will enter thesubstrate 40 from the front side 45.

The substrate 40 includes a plurality of pixels, also referred to asradiation-sensing elements or light-sensing elements. Theradiation-sensing elements are operable to sense or detect radiationwaves projected toward the substrate 40. For the sake of providing anexample, two of such radiation-sensing elements are shown in FIG. 2 anddesignated with reference numerals 50 and 51, though it is understoodthat any number of radiation-sensing elements may be formed in thesubstrate 40 to implement the image sensor device 30.

The radiation-sensing elements 50-51 are formed by performing aplurality of ion implantation processes on the substrate 40. Forexample, N+ implants, array-N-well implants, and deep-array-N-wellimplants may be performed. The ion implantation processes may includemultiple implant steps and may use different dopants, implant dosages,and implantation energies. The ion implantation processes may also usedifferent masks that have different patterns and opening sizes. Theradiation-sensing elements 50-51 may also be formed in a doped well (notillustrated) having an opposite doping polarity as the substrate 40.

In one embodiment, the radiation-sensing elements 50-51 includephotodiodes. In other embodiments, the radiation-sensing elements 50-51may include pinned photodiodes (PPD), photogates, reset transistors,source follower transistors, transfer transistors, or other suitabledevices.

The radiation-sensing elements 50-51 are also separated by isolationstructures, for example isolation structures 70, 71, and 72. Theisolation structures 70-72 may include shallow trench isolation (STI) ordeep trench isolation (DTI) devices. The STI or DTI devices are formedby etching openings (or trenches) in the substrate 40 and thereafterfilling the openings with a suitable material. This suitable materialhas a refractive index value that is less than a refractive index valueof silicon, which is approximately 4. In other words, the substrate 40is optically denser than the STI or DTI devices. In an embodiment, theSTI or DTI devices include silicon oxide, which has a refractive indexvalue of approximately 1.46. In another embodiment, the STI or DTIdevices include silicon nitride, which has a refractive index value ofapproximately 2.05. In yet another embodiment, the STI or DTI devicesinclude air, which has a refractive index value of approximately 1.

The isolation structures 70-72 may also each include a doped well thatsurrounds the STI or DTI device. The isolation structures 70-72 serve toprevent or substantially reduce cross-talk between adjacentradiation-sensing elements, such as radiation-sensing elements 50-51.The cross-talk may be electrical, or optical, or both. If left unabated,the cross-talk will degrade the performance of the image sensor device30.

In addition, although not illustrated for the sake of simplicity,transistor devices may be formed in the substrate 40. For example, ametal-oxide-semiconductor field-effect transistor (MOSFET) device may beformed in the substrate 40. The MOSFET device may have a gate, a source,and a drain. The source may be coupled to the radiation-sensing elements50 or 51. The gate and the drain may be coupled to external devicesthrough vias or contacts of an interconnect structure that will bediscussed later. The image sensor device 30 may further includeadditional circuitry and input/outputs adjacent to the pixels forproviding an operation environment for the pixels (such as pixels 50-51)and for supporting external communication with the pixels.

Still referring to FIG. 2, an interconnect structure 90 is formed overthe substrate 40. The interconnect structure 90 includes a plurality ofpatterned dielectric layers and conductive layers that provideinterconnections (e.g., wiring) between the various doped features,circuitry, and input/output of the image sensor device 30. Theinterconnect structure 90 includes an interlayer dielectric (ILD) and amultilayer interconnect (MLI) structure formed in a configuration suchthat the ILD separates and isolates each MLI structure from other MLIstructures. The MLI structure includes contacts, vias and metal linesformed on the substrate 40.

In one example, the MLI structure may include conductive materials suchas aluminum, aluminum/silicon/copper alloy, titanium, titanium nitride,tungsten, polysilicon, metal silicide, or combinations thereof, beingreferred to as aluminum interconnects. Aluminum interconnects may beformed by a process including physical vapor deposition (PVD), chemicalvapor deposition (CVD), or combinations thereof. Other manufacturingtechniques to form the aluminum interconnect may includephotolithography processing and etching to pattern the conductivematerials for vertical connection (via and contact) and horizontalconnection (conductive line). Alternatively, a copper multilayerinterconnect may be used to form the metal patterns. The copperinterconnect structure may include copper, copper alloy, titanium,titanium nitride, tantalum, tantalum nitride, tungsten, polysilicon,metal silicide, or combinations thereof. The copper interconnect may beformed by a technique including CVD, sputtering, plating, or othersuitable processes.

A dielectric layer 100 is then formed over the interconnect structure90. In an embodiment, the dielectric layer 100 includes a siliconnitride material. The dielectric layer 100 can be formed by a depositionprocess known in the art such as CVD, PVD, atomic layer deposition(ALD), or combinations thereof. The dielectric layer 100 has a thickness110. In an embodiment, the thickness 110 is in a range from about 2000Angstroms to about 4000 Angstroms.

A patterned photoresist layer is formed over the dielectric layer 100.The patterned photoresist layer has a plurality of photoresist portionsthat are separated by a plurality of openings. For the sake ofsimplicity, photoresist portions 120-121 and openings 130-132 areillustrated herein. The patterned photoresist layer is formed by firstdepositing a layer of photoresist material over the dielectric layer100, for example through a spin coating process. Thereafter, aphotolithography process known in the art is performed to pattern thephotoresist material into the photoresist portions 120-121. Thephotolithography process may include a plurality of masking, exposing,baking, developing, and rinsing processes. The photoresist portions120-121 each have a width (or a lateral dimension) 140. The photoresistportions 120-121 are also partially aligned vertically with theradiation-sensing elements 50-51. It is understood, however, that thephotoresist portions 120-121 may not have the same width values as theradiation-sensing elements 50-51, even though they are at leastpartially aligned.

Referring now to FIG. 3, a wet etching process is performed to etch theopenings 130-132 through the dielectric layer 100. The patternedphotoresist portions 120-121 are used as protective masks in thisprocess. Consequently, dielectric portions 100-101 are formed. Thedielectric portions 100-101 have approximately rectangular shapes andeach have a lateral dimension that is equal to the width 140 (which wasthe width of the photoresist portions 120-121). Since the photoresistportions 120-121 were approximately aligned with the radiation-sensingelements 50-51, respectively, the dielectric portions 100-101 are alsoapproximately aligned with the radiation-sensing elements 50-51,respectively.

Referring now to FIG. 4, the photoresist portions 120-121 are removed ina stripping or ashing process known in the art. Thereafter, an annealingprocess 160 is performed on the semiconductor device 30 to melt thedielectric portions 100-101. The annealing process 160 is performedusing the following process parameters:

-   -   a laser source that is an ultra-violet light with a wavelength        that is in a range from about 300 nanometers to about 600        nanometers;    -   an annealing duration that is in a range from about 150        nanoseconds to about 450 nanoseconds; and    -   an annealing energy that is in a range from about 0.5        mili-joules to about 2.5 mili-joules.

The annealing process 160 melts the dielectric portions 100-101 so thatthey turn from a solid form into a liquid form and thus undergo are-shaping process. Referring to FIG. 5, when the annealing process 160is over and the melted dielectric portions 100-101 are cooled, theybecome re-shaped dielectric portions 100A and 101A. The re-shapeddielectric portions 100A and 101A each have substantially the same width140 as the dielectric portions 100-101 before the annealing process 160is performed.

The re-shaped dielectric portions 100A and 101A also each have a height170 (or a vertical dimension) that is greater than the thickness 110(shown in FIG. 2) of the dielectric layer 100, which is the height ofthe dielectric portions 100-101 before being melted. In an embodiment, aratio between the height 170 and the thickness 110 is in a range fromabout 1.5 to about 2.5, for example, about 1.75. Stated differently, there-shaped dielectric portions 100A-101A are taller than the originalun-melted dielectric portions 100-101 by anywhere from about 1.5 timesto 2.5 times.

As can be seen from FIG. 5, the re-shaped dielectric portions 100A and101A each have a pointy or angular tip 180 (or a tip portion). Thepointy tip 180 is protruding at the tallest region of an upper surfaceof the re-shaped dielectric portions 100A and 101A. The pointy tip 180is formed as a result of the dielectric portions 100A and 101A havingbeen melted into liquid form and cooled into a solid form again. Thepointy tip 180 has an angle 190. In an embodiment, the angle 190 is in arange from about 100 degrees to about 120 degrees. An alternative way ofcharacterizing the pointy tip 180 is by drawing an imaginary horizontalline 200 that intersects the highest point of the pointy tip 180. Thisimaginary horizontal line 200 forms two angles 210 and 211 with there-shaped dielectric portions 100A and 101A on either side of the pointytips 180. The angles 210-211 are each in a range from about 30 degreesto about 40 degrees.

It is understood that the shape and geometry of the re-shaped dielectricportions 100A and 101A may be adjusted by tuning the values of the width140 (through the photolithography process that forms the patternedphotoresist portions 120, shown in FIG. 3) and the thickness 110 of thedielectric layer 100 (shown in FIG. 2, which is the height of theun-melted dielectric portions 100-101).

The re-shaped dielectric portions 100A and 101A serve as micro-lenses tohelp focus incoming radiation such as light. The pointy or sharp profileof the re-shaped dielectric portions 100A and 101A help improve theradiation-focusing performance. This will be discussed in more detaillater.

Referring now to FIG. 6, a dielectric material 230 is formed over there-shaped dielectric portions 100A and 101A. The dielectric material 230fills the openings 130-132 (shown in FIG. 5). The dielectric material230 includes a dielectric material different from the re-shapeddielectric portions 100A and 101A. For example, in an embodiment wherethe dielectric portions 100A and 101A include silicon nitride, then thedielectric material 230 may include silicon oxide or siliconoxy-nitride. The dielectric material 230 is then planarized by a processknown in the art to achieve a smooth and flat upper surface, for exampleby a chemical-mechanical-polishing (CMP) process.

Thereafter, a color filter layer containing a plurality of color filtersis formed over the planarized layer 230. For the sake of simplicity,only two of such color filters 250 and 251 are shown in FIG. 6. Thecolor filters 250 and 251 can support the filtering of radiation waveshaving different wavelengths, which may correspond to different colors,such as primary colors including red, green, and blue, or complementarycolors including cyan, yellow, and magenta. The color filters 250 and251 may also be positioned such that the desired incident lightradiation is directed thereon and therethrough. For example, the colorfilter 250 may filter the incident radiation such that only red lightreaches the radiation-sensing element 50. The color filter 251 mayfilter the incident radiation such that only green light reaches theradiation-sensing element 51. The color filters 250 and 251 may includea dye-based (or pigment based) polymer or resin to achieve the filteringof specific wavelength bands.

After the color filter layer is formed, a top micro-lens layer is formedover the color filter layer. For the sake of simplicity, only two ofsuch top micro-lenses 260 and 261 are illustrated in FIG. 6. The topmicro-lenses 260-261 help direct radiation toward the radiation-sensingelements 50-51, respectively. The top micro-lenses 260-261 may bepositioned in various arrangements and have various shapes depending ona refractive index of material used for the top micro-lenses anddistance from the surface of the substrate 40. In an embodiment, the topmicro-lenses 260-261 each include an organic material, for example aphotoresist material, or a polymer material. The top micro-lenses260-261 are formed by one or more photolithography processes.

Although not specifically illustrated for the sake of simplicity, it isunderstood that an anti-reflective layer may be formed underneath thecolor filter layer, and a spacer material may be formed between thecolor filter layer and the top micro-lens layer. Also, while theelements shown in FIGS. 2-6 are not drawn to scale and may havedifferent proportions in production, it is understood that the topmicro-lens 260, the color filter 250, the re-shaped dielectric portion100A (functioning as an embedded micro-lens), and the radiation-sensingelement 50 are all at least partially aligned vertically with oneanother. The same is true for the top micro-lens 261, the color filter251, the re-shaped dielectric portion 100B (functioning as an embeddedmicro-lens), and the radiation-sensing element 51.

As discussed above, the re-shaped dielectric portions 100A and 101A eachfunction as an embedded micro-lens. Such embedded micro-lenses offeradvantages over existing micro-lenses, it being understood that otherembodiments of the embedded micro-lenses constructed according to thepresent disclosure may offer different advantages, and that noparticular advantage is required for all embodiments. One advantage isthat the formation of the embedded micro-lenses herein involves simplefabrication processes. For example, the embedded micro-lenses are formedby a combination of an etching process and an annealing process. Incomparison, traditional micro-lenses are often times formed by aplurality of deposition, patterning (which typically includes dryetching), baking, and ashing processes. These processes may need to berepeated quite a number of times before an acceptable shape and geometrycan be achieved for a traditional micro-lens. As such, the formation ofthe traditional micro-lenses are time consuming and expensive, whereasthe formation of the embedded micro-lenses herein is quick and cheap.

Another advantage offered by the embedded micro-lenses herein isimproved light-focusing performance. Micro-lenses constructed bytraditional methods typically have a rounded profile for their uppersurfaces. The round profile results in a somewhat diminishedlight-focusing performance. In contrast, the embedded micro-lensesherein have sharp or angular profiles for their upper surfaces. Thesharp or angular profiles are more efficient at focusing the incominglight. As such, the embedded micro-lenses herein have improvedlight-focusing performance over the traditional micro-lenses.Furthermore, micro-lenses formed by the traditional methods typicallyhave rough surfaces due to the plurality of dry etching processesperformed. In comparison, the embedded micro-lenses herein are notformed by any dry etching process, and consequently have smoothersurfaces. The smoother surfaces also translate into betterlight-focusing performance.

One of the broader forms of the present disclosure is a semiconductordevice. The semiconductor device includes: a radiation-sensing elementformed in a substrate; a transparent layer formed over the substrate;and a micro-lens embedded in the transparent layer, wherein themicro-lens has a pointy tip portion.

Another one of the broader forms of the present disclosure is an imagesensor device. The image sensor device includes: a pixel located in asubstrate; a first micro-lens embedded in a layer over the substrate, afirst upper surface of the first micro-lens having an angular tip; acolor filter located over the layer; and a second micro-lens locatedover the color filter, a second upper surface of the second micro-lenshaving an approximately rounded profile; wherein the pixel, the firstmicro-lens, the color filter, and the second micro-lens are all at leastpartially aligned with one another.

Another one of the broader forms of the present disclosure is a method.The method includes: forming a radiation-sensing element in a substrate;forming a patterned dielectric layer over the substrate, the patterneddielectric layer including a plurality of dielectric portions separatedby a plurality of openings; and performing a laser annealing process onthe patterned dielectric layer in a manner such that each of thedielectric portions are melted and re-shaped, the re-shaped dielectricportions each having a pointy tip.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: aradiation-sensing element formed in a substrate; a transparent layerformed over the substrate; and a micro-lens embedded in the transparentlayer and positioned to focus received radiation onto theradiation-sensing element situated beneath the micro-lens, wherein themicro-lens has a curved upper surface with an angular tip with a pointedprofile.
 2. The semiconductor device of claim 1, further including: acolor filter layer formed over the transparent layer; and a furthermicro-lens formed over the color filter layer.
 3. The semiconductordevice of claim 2, wherein the micro-lens embedded in the transparentlayer and the further micro-lens are substantially aligned.
 4. Thesemiconductor device of claim 2, wherein the micro-lens embedded in thetransparent layer and the further micro-lens include differentmaterials.
 5. The semiconductor device of claim 4, wherein themicro-lens embedded in the transparent layer includes a dielectricmaterial, and the further micro-lens includes an organic material. 6.The semiconductor device of claim 1, wherein the angular tip has anangle that is in a range from about 100 degrees to about 120 degrees. 7.The semiconductor device of claim 1, wherein the semiconductor device isa front-side illuminated image sensor.
 8. An image sensor device,comprising: a pixel located in a substrate; a first micro-lens embeddedin a layer over the substrate, a first curved upper surface of the firstmicro-lens having an angular tip with a pointed profile; a color filterlocated over the layer; and a second micro-lens located over the colorfilter, a second upper surface of the second micro-lens having anapproximately rounded profile; wherein the pixel, the first micro-lens,the color filter, and the second micro-lens are all at least partiallyaligned with one another such that the first micro-lens focuses lightonto the pixel.
 9. The image sensor device of claim 8, wherein the firstand second micro-lenses are formed over the substrate in a verticaldirection; and wherein the tip of the first micro-lens forms an anglewith an imaginary line in a horizontal direction, the angle beingbetween about 30 degrees and about 40 degrees.
 10. The image sensordevice of claim 8, wherein: the first micro-lens includes a dielectricmaterial; and the second micro-lens includes an organic material. 11.The image sensor device of claim 8, wherein the image sensor device is afront-side illuminated image sensor.
 12. A method, comprising: forming aradiation-sensing element in a substrate; forming a patterned dielectriclayer over the substrate, the patterned dielectric layer including aplurality of dielectric portions separated by a plurality of openings;and performing a laser annealing process on the patterned dielectriclayer in a manner such that each of the dielectric portions are meltedand re-shaped, the re-shaped dielectric portions each having a curvedupper surface with an angular tip with a pointed profile.
 13. The methodof claim 12, wherein the forming the patterned dielectric layer iscarried out in a manner so that the dielectric portions each have asubstantially rectangular shape before the performing the laserannealing process.
 14. The method of claim 12, wherein the dielectricportions each includes a silicon nitride material.
 15. The method ofclaim 12, further including, after the performing the laser annealingprocess: forming a transparent material over the re-shaped dielectricportions; planarizing the transparent material to form a transparentlayer with the re-shaped dielectric portions embedded therein; forming acolor filter layer over the transparent layer; and forming a pluralityof micro-lenses over the color filter layer.
 16. The method of claim 15,wherein each of the micro-lenses is aligned with one of the re-shapeddielectric portions, the re-shaped dielectric portions each serving asan embedded micro-lens within the transparent layer.
 17. The method ofclaim 12, wherein the performing the laser annealing process is carriedout in a manner so that the angular tip has an angle that is in a rangefrom about 100 degrees to about 120 degrees.
 18. The method of claim 12,wherein the performing the laser annealing process is carried out usingthe following process parameters: a laser source that is an ultra-violetlight with a wavelength that is in a range from about 300 nanometers toabout 600 nanometers; an annealing duration that is in a range fromabout 150 nanoseconds to about 450 nanoseconds; and an annealing energythat is in a range from about 0.5 mili-joules to about 2.5 mili-joules.19. The method of claim 12, wherein: the forming the patterneddielectric layer is carried out in a manner so that each of thedielectric portions has a first width and a first height prior to beingannealed; and the performing the annealing process is carried out in amanner so that each of the re-shaped dielectric portions has a secondwidth and a second height, the second width being approximately equal tothe first width, and the second height being greater than the firstheight.
 20. The method of claim 19, wherein a ratio of the second heightand the first height is in a range from about 1.5 to about 2.5.